Support python virtualenv on w32 (Bug#24464)

Support python virtualenv on w32 (Bug#24464)

According to the virtualenv docs only POSIX systems follow the
structure "/path/to/venv/bin/", while windows systems use
"/path/to/venv/Scripts" for the location of the binary files, most
importantly including the python interpreter (see:
https://virtualenv.pypa.io/en/stable/userguide/#windows-notes).
* lisp/progmodes/python.el (python-shell-calculate-exec-path): Use the
"/path/to/venv/Scripts" for `windows-nt' machines.

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